This is the 6819th most frequent Chinese word.
洞中
"洞中" means "inside the cave."
In this context, '洞中' is used to describe the location 'inside the cave'.
洞中藏着许多宝物。
Inside the cave, there are many treasures hidden.
Here, '洞中' is used to denote a place where an action happens, i.e., 'inside the cave'.
他们进入洞中寻求庇护。
They entered the cave to seek shelter.
In this sentence, '洞中' signifies the source location 'inside the cave' of a reflected image.